The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
As an alternative to the internal combustion engine, automotive manufacturers have developed hybrid powertrains that include both an electric traction machine and an internal combustion engine. During operation, vehicles including the hybrid powertrain use one or both of the power sources to improve efficiency.
Hybrid vehicles include either a parallel drivetrain configuration or a series drivetrain configuration. In the parallel hybrid vehicle, the electric machine works in parallel with the engine to combine the power and range advantages of the engine with the efficiency and the electrical regeneration capability of the electric machine. In the series hybrid vehicle, the engine drives a generator to produce electricity for the electric machine, which drives a transaxle. This allows the electric machine to assume some of the power responsibilities of the engine, thereby permitting the use of a smaller and more efficient engine. Additionally, for either described hybrid configuration, the engine may be turned off while the vehicle is stopped and the driver's foot remains on the brake pedal. This is done to conserve fuel—increasing the duration of engine stoppage while the vehicle is at rest increases the hybrid fuel economy benefit.
Some hybrid vehicles include a vacuum driven brake booster that reduces the brake pedal effort required to achieve a desired vehicle braking force. These hybrid vehicles use the engine's intake manifold as a source for the vacuum which is stored in the brake booster. When the engine is turned off for hybrid operation, a finite level of vacuum is held in the brake booster which is depleted as the brake pedal is modulated. Hybrid vehicles require sufficient brake booster vacuum levels during engine off operation to maintain brake assist. If brake booster vacuum falls below a threshold during engine off hybrid operation, the engine will start so that brake booster vacuum can be replenished. Normally brake booster vacuum is depleted via brake modulation, but a leaky brake booster system can also cause brake vacuum to fall below the engine start threshold preventing or shortening hybrid engine off operation. Since this failure mode results in an impact to emissions and fuel economy, the brake booster system should be diagnosed for leaks.